shm_toc.c

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/*-------------------------------------------------------------------------
 *
 * shm_toc.c
 *    shared memory segment table of contents
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/storage/ipc/shm_toc.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "port/atomics.h"
#include "storage/shm_toc.h"
#include "storage/spin.h"
 
typedef struct shm_toc_entry
{
    uint64      key;            /* Arbitrary identifier */
    Size        offset;         /* Offset, in bytes, from TOC start */
} shm_toc_entry;
 
struct shm_toc
{
    uint64      toc_magic;      /* Magic number identifying this TOC */
    slock_t     toc_mutex;      /* Spinlock for mutual exclusion */
    Size        toc_total_bytes;    /* Bytes managed by this TOC */
    Size        toc_allocated_bytes;    /* Bytes allocated of those managed */
    uint32      toc_nentry;     /* Number of entries in TOC */
    shm_toc_entry toc_entry[FLEXIBLE_ARRAY_MEMBER];
};
 
/*
 * Initialize a region of shared memory with a table of contents.
 */
shm_toc *
shm_toc_create(uint64 magic, void *address, Size nbytes)
{
    shm_toc    *toc = (shm_toc *) address;
 
    Assert(nbytes > offsetof(shm_toc, toc_entry));
    toc->toc_magic = magic;
    SpinLockInit(&toc->toc_mutex);
 
    /*
     * The alignment code in shm_toc_allocate() assumes that the starting
     * value is buffer-aligned.
     */
    toc->toc_total_bytes = BUFFERALIGN_DOWN(nbytes);
    toc->toc_allocated_bytes = 0;
    toc->toc_nentry = 0;
 
    return toc;
}
 
/*
 * Attach to an existing table of contents.  If the magic number found at
 * the target address doesn't match our expectations, return NULL.
 */
shm_toc *
shm_toc_attach(uint64 magic, void *address)
{
    shm_toc    *toc = (shm_toc *) address;
 
    if (toc->toc_magic != magic)
        return NULL;
 
    Assert(toc->toc_total_bytes >= toc->toc_allocated_bytes);
    Assert(toc->toc_total_bytes > offsetof(shm_toc, toc_entry));
 
    return toc;
}
 
/*
 * Allocate shared memory from a segment managed by a table of contents.
 *
 * This is not a full-blown allocator; there's no way to free memory.  It's
 * just a way of dividing a single physical shared memory segment into logical
 * chunks that may be used for different purposes.
 *
 * We allocate backwards from the end of the segment, so that the TOC entries
 * can grow forward from the start of the segment.
 */
void *
shm_toc_allocate(shm_toc *toc, Size nbytes)
{
    volatile shm_toc *vtoc = toc;
    Size        total_bytes;
    Size        allocated_bytes;
    Size        nentry;
    Size        toc_bytes;
 
    /*
     * Make sure request is well-aligned.  XXX: MAXALIGN is not enough,
     * because atomic ops might need a wider alignment.  We don't have a
     * proper definition for the minimum to make atomic ops safe, but
     * BUFFERALIGN ought to be enough.
     */
    nbytes = BUFFERALIGN(nbytes);
 
    SpinLockAcquire(&toc->toc_mutex);
 
    total_bytes = vtoc->toc_total_bytes;
    allocated_bytes = vtoc->toc_allocated_bytes;
    nentry = vtoc->toc_nentry;
    toc_bytes = offsetof(shm_toc, toc_entry) + nentry * sizeof(shm_toc_entry)
        + allocated_bytes;
 
    /* Check for memory exhaustion and overflow. */
    if (toc_bytes + nbytes > total_bytes || toc_bytes + nbytes < toc_bytes)
    {
        SpinLockRelease(&toc->toc_mutex);
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of shared memory")));
    }
    vtoc->toc_allocated_bytes += nbytes;
 
    SpinLockRelease(&toc->toc_mutex);
 
    return ((char *) toc) + (total_bytes - allocated_bytes - nbytes);
}
 
/*
 * Return the number of bytes that can still be allocated.
 */
Size
shm_toc_freespace(shm_toc *toc)
{
    volatile shm_toc *vtoc = toc;
    Size        total_bytes;
    Size        allocated_bytes;
    Size        nentry;
    Size        toc_bytes;
 
    SpinLockAcquire(&toc->toc_mutex);
    total_bytes = vtoc->toc_total_bytes;
    allocated_bytes = vtoc->toc_allocated_bytes;
    nentry = vtoc->toc_nentry;
    SpinLockRelease(&toc->toc_mutex);
 
    toc_bytes = offsetof(shm_toc, toc_entry) + nentry * sizeof(shm_toc_entry);
    Assert(allocated_bytes + BUFFERALIGN(toc_bytes) <= total_bytes);
    return total_bytes - (allocated_bytes + BUFFERALIGN(toc_bytes));
}
 
/*
 * Insert a TOC entry.
 *
 * The idea here is that the process setting up the shared memory segment will
 * register the addresses of data structures within the segment using this
 * function.  Each data structure will be identified using a 64-bit key, which
 * is assumed to be a well-known or discoverable integer.  Other processes
 * accessing the shared memory segment can pass the same key to
 * shm_toc_lookup() to discover the addresses of those data structures.
 *
 * Since the shared memory segment may be mapped at different addresses within
 * different backends, we store relative rather than absolute pointers.
 *
 * This won't scale well to a large number of keys.  Hopefully, that isn't
 * necessary; if it proves to be, we might need to provide a more sophisticated
 * data structure here.  But the real idea here is just to give someone mapping
 * a dynamic shared memory the ability to find the bare minimum number of
 * pointers that they need to bootstrap.  If you're storing a lot of stuff in
 * the TOC, you're doing it wrong.
 */
void
shm_toc_insert(shm_toc *toc, uint64 key, void *address)
{
    volatile shm_toc *vtoc = toc;
    Size        total_bytes;
    Size        allocated_bytes;
    Size        nentry;
    Size        toc_bytes;
    Size        offset;
 
    /* Relativize pointer. */
    Assert(address > (void *) toc);
    offset = ((char *) address) - (char *) toc;
 
    SpinLockAcquire(&toc->toc_mutex);
 
    total_bytes = vtoc->toc_total_bytes;
    allocated_bytes = vtoc->toc_allocated_bytes;
    nentry = vtoc->toc_nentry;
    toc_bytes = offsetof(shm_toc, toc_entry) + nentry * sizeof(shm_toc_entry)
        + allocated_bytes;
 
    /* Check for memory exhaustion and overflow. */
    if (toc_bytes + sizeof(shm_toc_entry) > total_bytes ||
        toc_bytes + sizeof(shm_toc_entry) < toc_bytes ||
        nentry >= PG_UINT32_MAX)
    {
        SpinLockRelease(&toc->toc_mutex);
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of shared memory")));
    }
 
    Assert(offset < total_bytes);
    vtoc->toc_entry[nentry].key = key;
    vtoc->toc_entry[nentry].offset = offset;
 
    /*
     * By placing a write barrier after filling in the entry and before
     * updating the number of entries, we make it safe to read the TOC
     * unlocked.
     */
    pg_write_barrier();
 
    vtoc->toc_nentry++;
 
    SpinLockRelease(&toc->toc_mutex);
}
 
/*
 * Look up a TOC entry.
 *
 * If the key is not found, returns NULL if noError is true, otherwise
 * throws elog(ERROR).
 *
 * Unlike the other functions in this file, this operation acquires no lock;
 * it uses only barriers.  It probably wouldn't hurt concurrency very much even
 * if it did get a lock, but since it's reasonably likely that a group of
 * worker processes could each read a series of entries from the same TOC
 * right around the same time, there seems to be some value in avoiding it.
 */
void *
shm_toc_lookup(shm_toc *toc, uint64 key, bool noError)
{
    uint32      nentry;
    uint32      i;
 
    /*
     * Read the number of entries before we examine any entry.  We assume that
     * reading a uint32 is atomic.
     */
    nentry = toc->toc_nentry;
    pg_read_barrier();
 
    /* Now search for a matching entry. */
    for (i = 0; i < nentry; ++i)
    {
        if (toc->toc_entry[i].key == key)
            return ((char *) toc) + toc->toc_entry[i].offset;
    }
 
    /* No matching entry was found. */
    if (!noError)
        elog(ERROR, "could not find key " UINT64_FORMAT " in shm TOC at %p",
             key, toc);
    return NULL;
}
 
/*
 * Estimate how much shared memory will be required to store a TOC and its
 * dependent data structures.
 */
Size
shm_toc_estimate(shm_toc_estimator *e)
{
    Size        sz;
 
    sz = offsetof(shm_toc, toc_entry);
    sz = add_size(sz, mul_size(e->number_of_keys, sizeof(shm_toc_entry)));
    sz = add_size(sz, e->space_for_chunks);
 
    return BUFFERALIGN(sz);
}